Distributed threshold adjustment for high speed receivers

Abstract

According to one general aspect, a distributed threshold adjuster (DTA) may be interspersed between stages of a multistage amplifier to adjust the DC voltage of an input signal. The DTA may include an input signal terminal configured to receive the input signal. The DTA may also include a plurality of current sources configured to produce an adjustment current signal whose amperage is configured to be increased or decreased by fixed steps in order to adjust the DC voltage of the input signal. The DTA may include a control unit configured to selectively turn on or off the individual current sources of the plurality of current sources to select the amperage of the adjustment current signal. The DTA may further include an output terminal configured to produce an output signal, comprising a combination of the input signal and the adjustment current signal, to a stage of a multistage amplifier.

What is claimed is: 
     
       1. A distributed threshold adjuster (DTA) interspersed between a plurality of stages of a multistage amplifier and configured to adjust the DC voltage of an input signal to facilitate a decision by an analog-to-digital converter (ADC) comprising:
 an input signal terminal configured to receive the input signal; 
 a plurality of current sources configured to produce an adjustment current signal whose amperage is configured to be increased or decreased by fixed steps in order to adjust the DC voltage of the input signal,
 wherein each of the plurality of current sources are configured to provide a current that may be selectively added or subtracted from the input signal; 
 
 a control unit configured to selectively turn on or off the individual current sources of the plurality of current sources to select the amperage of the adjustment current signal; and 
 an output terminal configured to produce an output signal, comprising a combination of the input signal and the adjustment current signal, to a stage of a multistage amplifier. 
 
     
     
       2. The DTA of  claim 1 , wherein the output terminal is coupled with the input terminal and the current sources. 
     
     
       3. The DTA of  claim 1 , wherein the input signal terminal includes a coupling capacitor configured to pass the AC portion of the input signal to the output terminal. 
     
     
       4. The DTA of  claim 1 , wherein the input signal includes a pair of differential input signals, and the output signal includes a pair of differential output signals; and
 the DTA further includes a pair of resistors coupled in series with respect to one another and in parallel with respect to the output terminal, and 
 configured such that a common mode voltage (Vcm) exists between the pair of resistors. 
 
     
     
       5. The DTA of  claim 4 , wherein the plurality of current sources are coupled between the input terminal and the pair of resistors; and
 the plurality of current sources are coupled such that the output common mode voltage remains substantially constant regardless of the amperage of the adjustment current source. 
 
     
     
       6. The DTA of  claim 4 , wherein the plurality of current sources are configured to produce a positive adjustment current signal and a negative adjustment current signal;
 wherein the input signal includes a differential analog input signal; and 
 wherein the positive adjustment signal and the negative adjustment signal are interchangeably applied to the differential analog input signal. 
 
     
     
       7. The DTA of  claim 1 , wherein the plurality of current sources are configured to provide a substantially constant step size regardless of temperature. 
     
     
       8. The DTA of  claim 7 , wherein the plurality of current sources may be configured to produce a substantially fixed threshold step size regardless of process, supply voltage level and temperature variations. 
     
     
       9. A system comprising:
 a receiver configured to receive a pair of differential analog input signals; 
 a multistage amplifier configured to amplify the analog input signals by an amount of gain; 
 a plurality of distributed threshold adjusters (DTAs) interspersed between the stages of the multistage amplifier, and configured to adjust the DC voltages of the differential analog input signals to facilitate a decision by an analog-to-digital converter (ADC), and
 wherein each of the distributed threshold adjusters includes:
 a plurality of current sources configured to produce an adjustment current signal whose amperage is configured to be increased or decreased by fixed steps in order to adjust the DC voltage of the input signal, and 
 a control unit configured to selectively turn on or off the individual current sources of the plurality of current sources to select the amperage of the adjustment current signal; and 
 
 
 the ADC configured to convert the amplified analog input signals to a digital output signal. 
 
     
     
       10. The system of  claim 9 , wherein each of the DTAs further includes a pair of resistors
 coupled in series with respect to one another and in parallel with respect to the output terminals of the DTA, and 
 configured such that a common mode voltage (Vcm) exists between the pair of resistors. 
 
     
     
       11. The system of  claim 10 , wherein the plurality of current sources are coupled between the input terminals of the respective DTA and the pair of resistors; and
 the plurality of current sources are coupled such that the common mode voltage remains substantially constant regardless of the amperage of the adjustment current source. 
 
     
     
       12. The system of  claim 9 , wherein the plurality of current sources are configured to produce a positive adjustment current signal and a negative adjustment current signal;
 wherein the input signal includes a differential analog input signal; and 
 wherein the positive adjustment signal and the negative adjustment signal are interchangeably applied to the differential analog input signal. 
 
     
     
       13. The system of  claim 12 , wherein the absolute values of the amperage of the positive and negative adjustment current signals are substantially equal to maintain a relatively constant output common-mode voltage per amplifier stage and minimizing an operating-point induced gain variation per amplifier stage. 
     
     
       14. The system of  claim 9 , wherein the plurality of current sources are configured to provide a substantially constant step size regardless of temperature. 
     
     
       15. The system of  claim 14 , wherein the plurality of gain stages comprising the multistage amplifier chain are coupled with a proportional to absolute temperature (PTAT) biasing circuit to reduce temperature-induced variations in the gain per amplifier stage. 
     
     
       16. The system of  claim 14 , wherein the plurality of current sources employed to provide the threshold adjustment are coupled with either a calibrated or an uncalibrated bandgap biased resistor, depending on whether the corresponding load resistor through which the applied current flows is calibrated or uncalibrated, to provide a substantially constant step size regardless of temperature, supply-voltage changes or fabrication-induced variations. 
     
     
       17. The system of  claim 14 , wherein each of the plurality of current sources employed to provide the threshold adjustment includes a bandgap-based reference current source. 
     
     
       18. A method comprising:
 receiving a pair of differential analog input signals; 
 applying a plurality of gains to the differential analog input signals via a multistage amplifier; and 
 in between at least a portion of stages of the multistage amplifier, adjusting a DC voltage of the differential input signals by applying an adjustment current signal to the differential input signals that are the inputs of the respective stages of the multistage amplifier. 
 
     
     
       19. The method of  claim 18 , wherein applying an adjustment current signal comprises:
 producing, from a plurality of current sources, an adjustment current signal whose amperage is dynamically increased or decreased by fixed steps, 
 selectively turning on or off the plurality of current sources to dynamically select the amperage of the adjustment current signal, and 
 adding the adjustment current signal to the differential input signals. 
 
     
     
       20. The method of  claim 19 , wherein selectively turning on or off the plurality of current sources to dynamically select the amperage of the adjustment current signal includes:
 employing a multi-bit digital-to-analog converter to control whether or not each of the plurality of current sources is turned on or off; and 
 wherein each current source is controlled by a respective bit of the digital-to-analog converter.